Light grid arrangement using cyclically switched semiconductor light sources

Abstract

A light grid arrangement features a row of light emitting diodes 0 to 9 which are periodically switched on one after the other by a ring counter 11 and are arranged to direct their light through a monitoring region 23 towards an image forming optical means which is conveniently in the form of a concave mirror 24. The light beams are concentrated by the concave mirror 24 onto a photodetector 18 arranged behind a slot aperture 25 located at the focal point of the concave mirror. Missing pulses from the photodetector are detected by processing circuitry 26 to indicate obstruction of the light grid. The width of the slot aperture 25 restricts the width of each individual light beam so that only beams of light from the light emitting diodes that are generally parallel to the axis of the concave mirror are detected. In this way costly optical focussing elements are avoided as are the difficulties of aligning these optical elements with the other optical components of a system. The length of the monitored region can be increased as desired by placing a number of concave mirror and light emitting diode arrangements alongside one another.

We claim: 
     
       1. A light grid arrangement comprising a row of light emitting semiconductor elements, switching means for periodically and cyclically switching said light emitting semiconductor elements on one after the other, a monitoring region and image forming optical means which receives the light transmitted from the semiconductor elements through the monitoring region and which concentrates this light onto a photodetector, the arrangement further including a slot aperture located at the focal point of said image forming optical means, the slot of said slot aperture extending substantially at right angles to the row of light emitting semiconductor elements, the slot having a width so that the light leaving said light emitting semiconductor elements is trimmed in the direction of the row, whereby only a part of the light beam which is received by the image forming means can reach said photodetector which is arranged behind said slot aperture and which receives all the light passing through said slot. 
     
     
       2. A light grid arrangement in accordance with claim 1 wherein said light emitting semiconductor elements comprise light emitting diodes. 
     
     
       3. A light grid arrangement in accordance with claim 1 and wherein said photodetector comprises a single photodetector element. 
     
     
       4. A light grid arrangement according to claim 1 wherein the object distance is at a maximum 50 times as large as the focal length of said image forming optical means. 
     
     
       5. A light grid arrangement in accordance with claim 1 wherein said slot has a width so that only the useful light beams from neighbouring light emitting semiconductor elements overlap. 
     
     
       6. A light grid arrangement in accordance with claim 5 wherein the useful light beams of neighbouring light emitting semiconductor elements overlap by an amount in the range from 1/3 to 2/3 of their width at the image forming optical means. 
     
     
       7. A light grid arrangement according to claim 6 wherein the ratio of the width of said slot to the focal length of the image forming optical means preferably lies in the range from 0.2×10 -3  to 2×10 -3 . 
     
     
       8. A light grid arrangement according to claim 6 wherein the useful light beams of neighboring light emitting semiconductor elements overlap by an amount of about one-half of their width at the image-forming optical means. 
     
     
       9. A light grid arrangement according to claim 1 wherein said image forming optical means is of strip-like construction and extends parallel to said row of light emitting semiconductor elements. 
     
     
       10. A light grid arrangement according to claim 1 wherein said image forming optical means comprises a concave mirror. 
     
     
       11. A light grid arrangement according to claim 10 wherein said concave mirror is so tilted about its longitudinal axis that the photodetector and the aperture lie outside of the monitoring region. 
     
     
       12. A light grid arrangement according to claim 1 including a plurality of said image forming optical means with slot aperture and photodetector arranged directly alongside one another. 
     
     
       13. A light grid arrangement according to claim 12 and characterized in that neighbouring image forming optical means directly contact one another. 
     
     
       14. A light grid arrangement according to claim 12 wherein the photodetectors are connected in parallel to electronic processing circuitry. 
     
     
       15. A light grid arrangement in accordance with claim 12 including an additional light emitting semiconductor element provided between neighbouring rows of light emitting semiconductor elements whereby to direct light substantially at the seam between neighbouring image forming optical means. 
     
     
       16. A light grid arrangement according to claim 1 wherein the spacing of the light emitting semiconductor elements decreases in the outward direction away from the optical axis of the image forming optical means.